Physiology
Cough is a physiological reflex that helps clear secretions and foreign elements from the airways. The cough reflex is triggered by stimulation of mechano-receptors in the larynx, trachea, main carina and central airways. The afferent sensory signals are transmitted via pharyngeal branches of the glossopharyngeal nerve and by sensory fibers of the vagus nerve to the medulla oblongata. The signals are processed and a motor sequence of muscle activation is generated to create the cough.
Each cough starts with an inspiration, followed by closure of the glottis while the expiratory muscles (rectus abdominis and the intercostals muscles) contract. This contraction against a closed glottis rapidly elevates the intra-thoracic pressure. The glottis then opens and a burst of exhaled air flows through the trachea and out the mouth. The balance of forces on the tracheal wall, which includes the Bernoulli Effect, creates a net negative transmural pressure and dynamic tracheal collapse. As the trachea gets narrower, the speed of the air molecules through it increases, which further increases the negative transmural pressure and promotes additional collapse. The speed of air in the trachea during a cough may reach 100 m/s. The rapid gas movement along the tracheal walls creates substantial sheer forces, which help dislodge particles and secretions from the wall and into the air stream. These elements are then swept away with the gas outside of the airways through the glottis.
Each cough may have one or more components, or bursts for a single inspiration. Each burst is terminated by one of the following three mechanisms: (a) re-closure of the glottis, (b) termination of the expiratory muscles activity, or (c) exhaustion of the available gas flow reserve in the lung.
Patho-Physiology
While the cough reflex is an important component of the lung defense mechanisms, it is often a source of annoyance and concern to the patient. Chronic cough (i.e., frequent coughing that lasts for 2 weeks or more) may be an important sign of a lung disease (e.g., asthma, chronic bronchitis, malignancy, bronchiectasis, lung fibrosis or TB). There are, however, many cases where persistent cough is self-perpetuating, so that each cough spell applies such forces to the airway mucosa that stimulates and triggers a subsequent paroxysm, even in the absence of secretions or foreign bodies in the airways (“dry cough”).
Epidemiology
The prevalence of cough is high, leading to more visits to Drs' offices than any other single clinical symptom or sign. It is estimated that as many as 3% of all office visits to primary care physicians in the US are due to cough. Consequently, the market for cough remedies is huge, extending from various natural herbs and aromatic syrups to narcotic habit-forming drugs (e.g. codeine). The total market of antitussive medications in the US alone is estimated in Billions of $US.
Cough Detection
With cough being such a common problem, the need for methods and devices for objective detection and quantification of cough is obvious. Cough detection is needed in order to assess the severity of cough in an individual patient, to determine the association between coughing and certain offending conditions, such as occupational exposure, and for the evaluation and validation of the effectiveness of new and old cough remedies.
There have been several methods for detection of cough and for the creation of a log of coughing activity in a person or an animal. The simplest and, to this time, the most accurate method of detecting cough is by one or more trained observers who are either present in person with the index patient, or are listening and observing a video tape recording of the patient. As such, this method is often used as a reference (“gold standard”) for validation of an automated cough detection device.
Automated cough detection has been attempted and the following is a brief description of the available prior art. These methods may be divided into those that use the cough sound alone and those that use the cough sound in combination with other signals. Cough detection by loud-sound detection and recording on a sound-activated tape (or digital) recorder have been used in research by Mori et Al. (1987) and Sestini at Al (1991) . The timing of the tape activation may also be recorded with each loud sound. The total recording time, or the number of recorder activations are used as a cough activity index. This method is sensitive, but lacks specificity since other loud noises in the environment may trigger the recording as well. Subsequent auditory screening of the recorded sounds by a trained observer, or an automated algorithm (see below) may improve the specificity of this method.
Earis and co-workers (1994, 2002, 2003) described digital signal processing methods including spectral analysis and voice analysis methods to evaluate loud sounds to differentiate between cough sounds and other sounds such as vocalization. These studies as well as studies by other researchers analyzed ambient sounds recorded with a single microphone.
Other methods to detect coughs use two or more signals. Bush et Al., described a method by which a microphone output is used in combination with the signals from Electromyograph (EMG) electrodes placed on the abdominal muscles. These muscles contract in order to generate the elevated expiratory intra-thoracic pressure needed for an effective cough. Salmi et Al (Chest, 1988) used a static charge-sensitive bed to detect the cough-induced fast movements of the body and an ambient microphone output. Gavriely N. described in U.S. Pat. Nos. 6,261,238 and 6,168,568 a method in which the loud output of a microphone alongside a simultaneous sudden motion of the chest detected by a chest motion detector such as an electrical impedance plethysmograph are used as the first phase of cough detection algorithm.